1. Field of the Invention
The present invention relates to an information recording method and an optical disc apparatus which record information onto an optical disc by use of light.
2. Descriptions of the Related Arts
An optical disc formatted in accordance with a ZCLV (Zoned Constant Linear Velocity) scheme is divided into a plurality of zones in its radius direction, and recording/reproducing of information in each zone is performed by a CAV (Constant Angular Velocity) scheme. A frequency of a recording/reproducing signal is constant, and the ZCLV scheme has a feature in that information can be recorded densely even in an external periphery of a disc like in an inner periphery thereof In a DVD-RAM disc, this ZCLV scheme is adopted, and a high density recording of 2.6 GB is achieved in a first generation, and a high density recording of 4.7 GB is achieved in a second generation. In this CAV scheme, since a linear velocity is made to be constant in both of the inner and outer peripheries of the disc, a rotation number of the disc is made to be high when an optical head is above the inner periphery of the disc, and on the contrary the rotation number of the disc is made to be low when the optical head is above the outer periphery of the disc. Accordingly, a seek operation is performed for the disc, not only movement of the optical head but also the rotation number of the disc must be controlled. For example, in the case of an optical disc having a diameter of 120 mm, the rotation number of the disc becomes half when the seek operation is performed from 25 mm radius to 50 mm radius on the disc, and much time is taken for a control of a spindle motor. Accordingly, time for controlling a rotation number of the spindle motor sometimes determines seek time. Furthermore, a problem that power consumption increases because of the control of the rotation number of the spindle motor also occurs. In Japanese Patent Laid-Open No. 11 (1999)-296858, disclosed is a method to adjust write power in accordance with actual linear velocity is disclosed even when an actual rotation number cannot fully follow a targeted rotation number immediately after a seek operation and the like, hence a linear velocity differs from a targeted velocity.
If the rotation number is made to be approximately equal over the inner and outer peripheries of the disc, the time for controlling the rotation number in performing the seek operation can be shortened, and the power consumption necessary for controlling the rotation number of the spindle motor can be reduced. Accordingly, disclosed is a method in which, to realize a high transfer rate in a reproduction of information, even when a rotation number is constant or changes a little, the reproduction of the information recorded on a disc formatted in accordance with a CLV scheme is performed by making a clock frequency low in an inner periphery where a linear velocity is low and the clock frequency high in an outer periphery where the linear velocity is high (Japanese Patent Laid-Open Nos. 6 (1994)-89506, 6 (1994)-12785 and the like). In recording information, as in Japanese Patent Laid-Open No. 8 (1996)-212691, disclosed is a method in which a rotation number of the disc is made to be constant, and a clock frequency of a recording signal is made to be changed depending on a linear velocity, thus enabling a high speed recording. In this recording method, since the linear velocity changes depending on a radius of the disc, recording conditions must be optimized in accordance with the linear velocity. According to Japanese Patent Laid-Open Nos. 8 (1996)-212691 and 10 (1998)-106025 and the like, recording conditions are optimized by changing properties of a recording medium depending on whether recording is performed for an inner periphery of the recording medium or for an outer periphery thereof. However, this recording medium is used exclusively for an optical disc of which rotation number is controlled to be constant or approximately constant, therefore uses of this recording medium are limited. As means for solving these problems, in Japanese Patent Laid-Open No. 10 (1998)-106008, disclosed is a method in which recording conditions are not optimized depending on the recording medium, but a recording mark having the most preferable shape is formed by changing power and a pulse width in recording information thereonto depending on a linear velocity.
Also in the ZCLV scheme in which a linear velocity is approximately constant, to achieve a high density recording, a scheme for changing a recording pulse width in forming a recording mark is adopted. As is written in p. 70 of Sixth Phase Change Recording Conference Abstracts, 1994, this ZCLV scheme is the one in which a recording pulse shape is varied with a length shorter than a clock unit depending on a precedent space and/or a subsequent space, (an adaptive type recording waveform control). This scheme can suppress an edge shift as little as possible, which originates from thermal interference along a track direction caused by a fact that a recording mark and a space are smaller than a light spot, and it is very effective for a mark edge recording. In the adaptive type recording waveform control, test writing is performed before information is recorded onto a data section, so that the optimized recording waveform and the optimized write power can be obtained in accordance with individual discs and operation circumstances. For example, in the case of a 4.7 GB DVD-RAM medium, a write test zone is provided in each of an inner periphery area and an outer periphery area (see FIG. 2), and the test writing is executed by use of these zones when necessary. When the linear velocity is approximately constant, the test writing has been executed at one linear velocity by use of the inner periphery area or/and the outer periphery area.
In general, when a linear velocity is made to be fast, time for passing a light spot on a recording mark on a disc is shortened. Accordingly, an amount of energy radiating onto the disc per unit time reduces, and a recording mark is hard to be heated. Therefore, laser power necessary for erase becomes high. Moreover, as the linear velocity becomes faster, a clock frequency becomes higher. Accordingly, information is recorded with write pulses having a narrower width, so that higher write power becomes necessary. A relation between the linear velocity and the radiation power is applied to all optical discs. In the case of a phase-change recording, it is important to change an increase of the erase power and the write power for the linear velocity. The reason is as follows.
In the phase-change recording, a recording layer is heated to its melting point, and thereafter cooled rapidly, whereby an amorphous recording mark is formed. A so-called re-crystallization region is formed in the periphery of the recording mark, the re-crystallization region being formed in such a manner that the recording mark is once heated to its melting point, and thereafter a crystal growth begins from a crystal nucleus around the recording mark during a cooling step to produce a crystal phase. It was made clear as a result of an investigation that a width of this re-crystallization region depends on a linear velocity during recording, and the width thereof becomes narrower as the linear velocity is faster. Accordingly, when recording is performed while varying the linear velocity, dependency of the width of the re-crystallization region on the linear velocity must be considered to obtain a certain recording mark width. This is because the phase-change recording is a recording principle in which a cooling step subsequent to the temperature elevation is more important, unlike a recording principle in which a mark shape is decided depending on temperature distribution like magneto-optical recording.
When the linear velocity becomes faster, a clock frequency becomes higher according to the linear velocity, and a window width Tw becomes shorter. Accompanied with shortening of the window width Tw, light emission time of the pulses becomes shorter, so that influences of a rise-up time of laser become larger. As shown in FIG. 3, when the linear velocity is slow, the clock frequency is low. Accordingly the influence of the rise-up time of the laser is a little, and hence a light emission pulse akin to a rectangular shape can be obtained. However, when the linear velocity is fast, the clock frequency becomes high, and the pulse drops before the laser outputs inherent power, so that sufficient power cannot be sometimes obtained substantially Accordingly, the optimized recording waveform needs to be obtained for each variable linear velocity respectively.
Form above-described viewpoints, when the recording is performed while varying the linear velocity, it is necessary to obtain the optimized recording power and recording waveform for the respective linear velocities by performing the test writing at these linear velocities. However, since in the DVD-RAM disc, the test writing area exists only in the inner and outer peripheries thereof, there has been a problem that the test writing can be performed only at the two kinds of the linear velocities, that is, only at the linear velocity on the inner periphery of the disc and at the linear velocity on the outer periphery thereof.